Recently, a concept of “wearable computing” using a constant-wearable device has been proposed. Particularly, a camera intended for constant wearing and constant capturing (hereinafter, described as a “wearable camera”) is capable of recording an experience of a wearer lively as it is, and various applications thereof can be considered.
One of features necessary for such a wearable camera is an angle of view comparable to a human visual field. Conventionally, for an optical system which allows such a wide angle of view, an optical system using a fisheye lens or a convex mirror has been used. Above all, an optical system using a parabolic mirror or a hyperboloidal mirror for the convex mirror has a feature of single viewpoint, that is, having properties that reflected light converges on a single point. For example, Patent Literature 1 discloses a configuration of an omnidirectional visual sensor having a single viewpoint as described above. With a configuration disclosed in Patent Reference 1, as shown in FIG. 1, light 205 proceeding to a focal point 202 of a hyperboloidal portion 201 of one of two sheets is reflected toward a focal point 204 of a hyperboloidal portion 203 of the other of the two sheets. Thus, it is possible to obtain an image having a single viewpoint by providing a mirror having a shape of the hyperboloidal portion 201 of the one of the two sheets and positioning a lens at the focal point 204 of the hyperboloidal portion 203 of the other one of the two sheets. Note that in the case of a set lens including a plurality of lenses, the same effect can be produced by positioning the front principal point of the lens group at the focal point 204 of the hyperboloidal portion 203 of the other one of the two sheets.
An advantage of having a single viewpoint is to allow a captured image to have the same projection characteristics as those of a general camera. This produces advantageous effects such as allowing applying, to the captured image, general image processing based on image geometry, or to convert the captured image into a general perspective projection image without distortions.
[Citation List]
[Patent Literature]
[PTL 1] Japanese Patent No. 2939087
[PTL 2] Japanese Unexamined Patent Application Publication No. 2007-264402